The light-emitting diode (LED) is a solid state semiconductor device. A structure of the light-emitting diode (LED) comprises a p-type semiconductor layer, an n-type semiconductor layer, and an active layer. The active layer is formed between the p-type semiconductor layer and the n-type semiconductor layer. The structure of the LED generally comprises III-V group compound semiconductor such as gallium phosphide (GaP), gallium arsenide (GaAs), or gallium nitride (GaN). The light-emitting principle of the LED is the transformation of electrical energy to optical energy. An external electrical current drives electrons provided from the n-type semiconductor layer and holes provided from the p-type semiconductor layer to combine near p-n junction of the active layer. Then, the LED emits light when the electrons and the holes combine. However, during the combination of electrons and holes, part of electrical energy becomes heat which affects optical-electrical characteristics of the LED, for example, decreases light-emitting efficiency.
To achieve high color rendering and high efficiency of lighting requirements of the LED, a red chip capable of emitting a red light, a blue chip capable of emitting a blue light and a phosphor are usually combined to emit a white light. But, when the external electrical current is injected into the LED, part of electrical energy becomes heat. When the electrical current is continuously injected into the LED, thermal heat continues to accumulate. The accumulated thermal heat causes the temperature of the LED increasing and the light-emitting efficiency of the LED decreasing, while the thermal heat impacts the light-emitting efficiency of the red chip more than that of the blue chip.
As shown in FIG. 1, when the external electrical current is injected into the LED, the temperature of the LED increases from an original temperature to a higher temperature, such as from 25° C. to 75° C. The photo decay dependence on temperature of the red chip is different from that of the blue chip, which leads to the color temperature of the LEDs at 25° C. being different from that of the LEDs at thermal equilibrium. The color temperature of the lighting apparatus therefore shifts and lighting apparatus can fail.
FIG. 1A illustrates a diagram of light intensity dependence on temperature of a conventional red chip. As shown in FIG. 1A, when the external electrical current is injected into the red chip, the temperature of the red chip increases from an original temperature to a higher temperature, such as from 25° C. to 85° C. or above, and the light intensity attenuates with increasing temperature. The attenuation rate of the light intensity versus temperature is approximately −0.87%/deg C. FIG. 1B illustrates a diagram of emission wavelength dependence on temperature of a conventional red chip. As shown in FIG. 1B, when the external electrical current is injected into the red chip, the temperature of the red chip increases from an original temperature to a higher temperature, such as from 25° C. to 85° C. or above, and the emission wavelength shifts towards long wavelength with increasing temperature. When the temperature increases from 25° C. to 100° C., the emission wavelength of the red chip shifts about 5.7 nm.
Generally, electronically controlling method is used to solve the color temperature differences of the LED at thermal equilibrium state and at initial current driving state. However, this method increases the manufacturing cost of LED bulb.